Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for seismic data acquisition using a receiver underwater.
Discussion of the Background
During the past years, the interest in developing new oil and gas production fields has dramatically increased. However, the availability of land-based production fields is limited. Thus, the industry has now extended drilling to offshore locations, which appear to hold a vast amount of fossil fuel. Offshore drilling is an expensive process. Thus, those engaged in such a costly undertaking invest substantially in geophysical surveys in order to more accurately decide where to drill in order to avoid a dry well.
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of the oil and/or gas reservoirs. Thus, providing a high resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 drags an array of acoustic detectors 12. Plural acoustic detectors 12 are disposed along a cable 14. Cable 14 together with its corresponding detectors 12 are sometimes referred to, by those skilled in the art, as a streamer 16. The vessel 10 may tow plural streamers 16 at the same time. The streamers may be disposed horizontally, i.e., lying at a constant depth z1 relative to a surface 18 of the ocean. Also, the plural streamers 16 may form a constant angle (i.e., the streamers may be slanted) with respect to the surface of the ocean as disclosed in U.S. Pat. No. 4,992,992, the entire content of which is incorporated herein by reference. FIG. 2 shows such a configuration in which all the detectors 12 are provided along a slanted straight line 14 making a constant angle α with a reference horizontal line 30. Alternatively, the streamers may have other shapes.
With reference to FIG. 1, the vessel 10 also drags a sound source 20 configured to generate an acoustic wave 22a. The acoustic wave 22a propagates downward and penetrates the seafloor 24, eventually being reflected by a reflecting structure 26 (reflector). The reflected acoustic wave 22b propagates upwardly and is detected by detector 12. For simplicity, FIG. 1 shows only two paths 22a corresponding to the generated acoustic wave. However, the acoustic wave emitted by the source 20 may be substantially a spherical wave, e.g., it propagates in all directions starting from the source 20. Parts of the reflected acoustic wave 22b (primary) are recorded by the various detectors 12 (the recorded signals are called traces) while parts of the reflected wave 22c pass the detectors 12 and arrive at the water surface 18. Since the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water surface acts as a mirror for the acoustic waves), the reflected wave 22c travels back towards the detector 12 as shown by wave 22d in FIG. 1. Wave 22d is normally referred to as a ghost wave because this wave is due to a spurious reflection. The ghosts are also recorded by the detector 12, but with a reverse polarity and a time lag relative to the primary wave 22b. 
Thus, every arrival of a marine seismic wave at detector 12, with the exception of the direct arrival, is accompanied by a ghost reflection. In other words, ghost arrivals trail their primary arrival and are generated when an upward traveling wave is recorded a first time on submerged equipment before being reflected at the surface-air contact. The now downward propagating reflected wave 22d is recorded a second time at detector 12 and constitutes the ghost. Primary and ghost signals are also commonly referred to as up-going and down-going wave fields.
The time delay between an event and its ghost depends entirely upon the depth of the receiver 12. It can be only a few milliseconds for streamers towed underwater (depths of less than 15 meters) or up to hundreds of milliseconds for deep Ocean Bottom Cable (OBC) and Ocean Bottom Node (OBN) acquisitions. The degenerative effect that the ghost arrival has on seismic bandwidth and resolution are known. In essence, interference between primary and ghost arrivals causes notches or gaps in the frequency content and these notches cannot be removed without the combined use of advanced acquisition and processing techniques.
An improvement to the conventional data acquisition is the use of a wide azimuth acquisition (WAZ). In a typical WAZ survey, two streamer vessels and multiple sources are used to cover a large sea area, and all sources and streamers are controlled at a uniform depth throughout the survey. The WAZ provides a better illumination of the substructure and thus a better final image. However, the presence of the ghosts in the acquired data still affects the final image due to the presence of notches as schematically illustrated next.
A notch centers at a frequency at which a distance between a detecting hydrophone (receiver) of the streamer and the water surface is equal to one-half of its wavelength. FIG. 3 illustrates the spectral difference from streamers towed at 10 m depth (see curve 40) and 20 m depth (see curve 42). FIG. 3 plots an amplitude of the recorded signal versus a corresponding frequency. A notch 44 is also illustrated in FIG. 3. Shallower-towed streamers increase the high-frequency content, but also attenuate the low frequencies because of stronger environmental noise. Deeper-towed streamers, enhance low frequencies, but also move the first spectral notch lower into the frequency band of interest.
Removing the ghost effect has been the subject of geophysical research for many years. Two methods have been developed that enhance the signal to noise ratio and frequency bandwidth compared to standard shallow towed spreads. One such method is the over-under acquisition and it is described in U.S. Pat. No. 7,372,769 (the entire content of which is incorporated by reference herein). In this method, the streamers are either towed as vertically aligned pairs, or towed with regular number of streamers on an upper layer and with a smaller number of streamers at a deeper layer. The shallow towed layer of streamers is used for better mid and upper frequencies in the survey, and the deeper towed layer of streamers is used for better low frequencies. In processing, these two data sets are combined for a better overall spectrum. However, this approach requires additional number of streamers and imposes another level of operation complexity that lead to lower acquisition efficiency.
The second method uses a dual sensor approach (U.S. Pat. No. 7,239,577, the entire content of which is incorporated herein by reference). This method uses velocity sensors (e.g., gimbaled geophones) that are co-located with pressure gradient sensors (hydrophones) in the streamer. Hence, the two sets of data are collected from the same location and used to compensate each other for subsequent data combination to remove the spectral notches. However, this approach needs special streamers each with twice as many sensors as the regular streamers.
Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks, and improve the accuracy of the final image.